@Override     
public boolean onKeyDown(int keyCode, KeyEvent event) {
           if (keyCode == KeyEvent.KEYCODE_BACK) {               moveTaskToBack(false);              
            return true;       
                }        
                   return super.onKeyDown(keyCode, event);  
                        }

主要是moveTaskToBack(false);方法，源码如下

/**      * Move the task containing this activity to the back of the activity      * stack.  The activity's order within the task is unchanged.      
 of a task; if true it will work for any activity in      *                a task.      *       * @return If the task was moved (or it was already at the      *         back) true is returned, else false.      */    
 public boolean moveTaskToBack(boolean nonRoot) {         
return ActivityManagerNative.getDefault().moveActivityTaskToBack(                     mToken, nonRoot);        
 } catch (RemoteException e) {          
    }    
   }